Text classification User Guide

[TobyRoseman]

The Text Classification Page of our User Guide needs some work.

At a minimum these should be done:

• [ ] "bag-of-words" link doesn't seem to take you to the right place.
• [ ] We have only this line of code classifier = model.classifier for "inspect the underlying classifier model using"
• [x] How do we get ratings_data.csv?
• [ ] Under "How it works" it says "Training a text classifier is really important when you want to tune the model to your data set to take advantage of vocabulary that is particular to your application." Is the usage of "Tune" still accurate?

[afranklin]

Handling ratings_data.csv as part of https://github.com/apple/turicreate/issues/97